Abstract: Increasingly stringent vehicle emission legislation necessitates increasingly efficient combustion engines. The use of direct injection in spark ignited engines improves engine efficiency and reduces CO2 emissions. Direct injection allows stratified combustion which can further improve engine efficiency in part load operation with the penalty of increased emissions of soot from fuel rich combustion and pool fires with gasoline. Hydrogen, methane and ethanol are alternative fuels which can reduce emissions in direct injection engines operating with homogeneous or stratified combustion and are available from renewable sources. This thesis consists of an investigation of the transient gas jets that are created from direct injection of gas fuels and the stratified combustion of E85 and main sources of soot formation. The transient structure of gas jets created during direct injection of gas fuels in conditions relevant to homogeneous and stratified engine operation was investigated using time resolved schlieren imaging of helium jets in a pressure chamber. The jets became self-similar with respect to the width to penetration length ratio after an initial transition. The jet self-similarity variable decreased with increasing injection to ambient pressure ratio. At the same pressure ratio the self-similarity scaled non linearly with the injection pressure and ambient pressure. The self-similarity parameter was larger than reported in the literature resulting in a smaller penetration length constant. The Turner model was accurate for the transient compressible jets investigated with a constant relative error in the predicted shape. Stratified combustion of E85 was studied in an optical engine in to determine the main sources of soot formation, how soot formation could be reduced and how soot oxidation could be increased. Time and spatially resolved imaging of Mie scattering of diffuse light, combustion luminosity, OH · ' chemiluminescence and soot luminescence along with cylinder pressure measurements were used to study combustion, flame propagation, soot formation and soot oxidation. Pool fires consisting of diffusion flames from combustion of liquid fuel films on the piston crown were the primary source of soot formation for a single injection of fuel. The liquid fuel films, pool fires and soot formation were reduced and soot oxidation increased by splitting a single injection into multiple injections and igniting the last fuel spray. The primary source of soot formation when the first of multiple fuel sprays was ignited was rich combustion in the gas phase.